The Seventeenth Amendment to the United States Constitution provides for the popular election of each state’s senators. Prior to the amendment (from 1913), the state legislatures chose United States senators. The majority party in a state legislature held the senate seats as prizes, sometimes awarded to the highest bidder. Corruption was so much the rule that this phrase became popular from at least 1899: “It is harder for a poor man to enter the United States Senate than for a rich man to enter Heaven.”
   
The phrase borrows from this saying in the New Testament: “I tell you the truth, it is hard for a rich man to enter the kingdom of heaven. Again I tell you, it is easier for a camel to go through the eye of a needle than for a rich man to enter the kingdom of God.”
 
The phrase became re-used in 2008-2009, when senator Barack Obama was elected president, senator Joe Biden was elected vice president, and senator Hillary Clinton became secretary of state. Illinois Governor Rod Blagojevich was accused of selling Obama’s Illinois senate seat.
   
   
Wikipedia: Seventeenth Amendment to the United States Constitution
The Seventeenth Amendment (Amendment XVII) to the United States Constitution was passed by the Senate on June 12, 1911, the House of Representatives on May 13, 1912, and ratified by the states on April 8, 1913. The amendment supersedes Article I, § 3, Clauses 1 and 2 of the Constitution, transferring Senator selection from each state’s legislature to popular election by the people of each state. It also provides a contingency provision enabling a state’s governor, if so authorized by his state’s legislature, to appoint a Senator in the event of a Senate vacancy until either a special or regular election to elect a new Senator is held.
   
Wikipedia: Eye of a needle
The eye of a needle is the section of a sewing needle formed into a loop for pulling thread, located at the end opposite the point. These loops are often shaped like an oval or an “eye”, hence the metaphor.

Eyes of needles are often notoriously small and difficult to thread, leading to an aphorism used in the religious texts of Judaism, Christianity, and Islam. These aphorisms are based on the impossibility of passing a large object or animal through the eye of a needle.
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Christianity
“The eye of a needle” is part of a phrase attributed to Jesus by the synoptic gospels:
 
...I tell you the truth, it is hard for a rich man to enter the kingdom of heaven. Again I tell you, it is easier for a camel to go through the eye of a needle than for a rich man to enter the kingdom of God.
 
The parallel versions appear in Matthew 19:23-24, Mark 10:24-25 and Luke 18:24-25.
 
The saying was a response to a young rich man who had asked Jesus what he needed to do in order to inherit eternal life. Jesus replied that he should keep the commandments. To which the man stated he had done. Jesus responded, “If you want to be perfect, go, sell your possessions and give to the poor, and you will have treasure in heaven. Then come, follow me.” The young man became sad and was unwilling to do this. Jesus then spoke this response, leaving his disciples astonished.
 
Some commentators have found it incredible to speak of a rich man’s chance of being saved as being harder than threading a camel through a literal sewing implement. Consequently the phrase has inspired various interpretations.
     
National Archives: The Direct Election of Senators
“It is harder for a poor man to enter the United States Senate than for a rich man to enter Heaven.”
Late-19th-century political aphorism
critiquing the Senate as a “millionaire’s club”
 
The Constitution, as it was adopted in 1788, made the Senate an assembly where the states would have equal representation. Each state legislature would elect 2 senators to a 6-year term. Late in the 19th century some state legislatures deadlocked over the election of a senator when different parties controlled different houses, and Senate vacancies could last months or years. In other cases, special interests or political machines gained control over the state legislature. Progressive reformers dismissed individuals elected by such legislatures as puppets and the Senate as a “millionaire’s club” serving powerful private interests.
 
One Progressive response to these concerns was the “Oregon system” which utilized a state primary election to identify the voters’ choice for senator while pledging all candidates for the state legislature to honor the primary’s result. Over half of the states adopted the “Oregon system,” but the 1912 Senate investigation of bribery and corruption in the election of Illinois Senator William Lorimer indicated that only a constitutional amendment mandating the direct election of senators by a state’s citizenry would allay public demands for reform.
   
6 January 1899, Nebraska State Journal (Lincoln, NE), pg. 8, col. 3:
He (Silas A. Holcomb, Nebraska governor—ed.) recommends that the national congress be memorialized to amend the constitution so that United States senators may be elected by a direct vote of the people. “It has been stated in congress that it is as difficult for a poor man to enter the senate of the United States as it is for a rich man to enter the kingdom of heaven.”
   
2 February 1900, Helena (MT) Independent, pg. 6:
“The impression prevails that it is as difficult for a poor man to enter the senate of the United States as for a rich man to enter the kingdom of heaven.”
(Speech in Congress by Mr. Corliss, of Michigan—ed.)
   
Google Books
The Election of Senators
By George H. Haynes
New York, NY: Henry Holt and Company
1906
Pg. 173:
Much of the prejudice is ill-founded, yet it is not without significance that, in the language of the street, the Senate is so often spoken of as the “millionaires’ club” while, even in Congressional debates, the proverb in the revised version runs: “It is harder for a poor man to enter the United States Senate than for a rich man to enter Heaven.”
     
SocialistWorker.org
One bad apple or a whole rotten barrel?
Rod Blagojevich’s caught-on-tape horse-trading stands only a few degrees of separation away from business as usual in U.S. politics.
December 18, 2008
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Amid all the outrage (manufactured and genuine) over Blagojevich’s apparent malfeasance, it’s easy to forget that for most of U.S. history, the process of selecting senators was even more corrupt. Only in 1914, after passage of the 17th Amendment to the Constitution, were senators elected by popular vote.
 
Before that, state legislatures voted for U.S. senators. And very often, senate candidates purchased those votes with money from their industrialist backers, or from their own pockets. This was the reality behind the 19th century paraphrase of a Bible passage: “It’s harder for a poor man to enter the U.S. Senate than for a rich man to enter the kingdom of heaven.”