Vice President Joe BidenJoe BidenThe Hill's 12:30 Report Dems see huge field emerging to take on Trump Lawmakers send McCain well wishes after cancer diagnosis MORE will headline a fundraiser for Sen. Mark UdallMark UdallDemocratic primary could upend bid for Colorado seat Picking 2018 candidates pits McConnell vs. GOP groups Gorsuch's critics, running out of arguments, falsely scream 'sexist' 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 GardnerInterior recommends preserving Colorado site's monument status Overnight Energy: Exxon sues feds over M fine | Deputy Interior pick advances | Oil concerns hold up Russia sanctions push Senate advances controversial Trump Interior nominee 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.