Vice President Joe BidenJoe BidenBiden: I regret not running for president Biden: 'McCain is right: Need select committee' for Russia With no emerging leaders, no clear message, Democrats flounder MORE will headline a fundraiser for Sen. Mark UdallMark UdallGorsuch's critics, running out of arguments, falsely scream 'sexist' Election autopsy: Latinos favored Clinton more than exit polls showed Live coverage: Tillerson's hearing for State MORE (D-Colo.) later this month.

He'll attend a May 27 fundraiser hosted by Udall and the Colorado Democratic Party, according to an invitation obtained by The Denver Post.

The Colorado Senate race became competitive with the entry of Rep. Cory GardnerCory GardnerRepeal of Obama drilling rule stalls in the Senate Colorado Dem at the center of Gorsuch confirmation fight Gorsuch sails on day one, but real test is Tuesday MORE (R-Colo.), a top GOP recruit, into the race earlier this year.

Udall has kept up a steady fundraising pace and ramped it up this past quarter, bringing in more than $2 million and closing out the quarter with $5.9 million cash on hand.

But Gardner reported raising $1.4 million, $1.2 million of that in the first month of his campaign, and had $2.1 million cash on hand at the end of the quarter. At that pace, Gardner is well on his way to catching up to Udall’s fundraising lead.

Biden's visit to the state is intended to help Udall maintain it, as he prepares for a tough fight this fall.