As he continues his campaign for the Republican presidential nomination, Rand Paul will begin facing pressure at home as a viable Democratic challenger has filed to run for his Senate seat.

Lexington Mayor Jim Gray filed for the Democratic nomination for the U.S. Senate before Tuesday's 4 p.m. deadline. Gray, the wealthy former CEO of a construction company, made history in 2010 when he was elected Kentucky's first openly gay mayor. Since then, he has wrangled the city's pension funds and signed an ordinance raising the local minimum wage to $10.10 an hour.

Yet Gray was not the party's first choice to take on Paul, who has been out of the state for months campaigning for president in Iowa and the early primary states of New Hampshire and South Carolina. Party leaders had planned for former auditor Adam Edelen, a dynamic speaker and seen as one of the state's future Democratic leaders, to begin putting pressure on Paul after the state elections in November.

But Edelen lost in a surprising upset to a little-known Republican state representative from Danville, ending his Senate campaign before it began. Kentucky Democrats have been in disarray ever since, with the state party chairman resigning and party leaders yet to pick a replacement. But Gray gives Democrats a savvy political veteran who has shown a willingness to use his own money to fuel his campaigns. He used more than $800,000 of his own money in 2010 to topple an incumbent mayor, and gave himself another $250,000 in his 2014 re-election campaign.

A self-funded candidate could compensate for the campaign's late start and help keep pace with Paul, who has a modest $1.4 million available in his Senate campaign account.

Paul is the only major party presidential candidate who is running for two offices. Republican Sen. Marco Rubio of Florida decided not to seek re-election to his Senate seat so he could focus on the presidential race. Paul ran into a legal problem in Kentucky, where a state law forbids candidates from appearing on the ballot twice in the same election. Kentucky Republicans decided to hold a presidential caucus on March 5 to get around that, with Paul donating $250,000 to cover the causes' expenses.

Paul has dropped in presidential polling, so much that he was bumped from the main stage of the latest televised debate in Iowa. But he has managed to make most of the votes in the U.S. Senate while keeping a busy campaign schedule, saying that shows his commitment to his job.