Del. Robert G. Marshall, a Prince William Republican and one of the most conservative voices in the House of Delegates, said Thursday that he will run for U.S. Senate this year.

Marshall, who narrowly lost the Republican nomination for U.S. Senate in 2008, will face former governor and senator George Allen, the heavy favorite to win the crowded GOP primary in June.

Marshall said that he is collecting signatures to qualify for the ballot and that he plans to start raising money and file papers to run in coming days. He registered his committee, Bob Marshall for Senate Inc., with the state Monday.

“I need to finish a few mechanical steps,” he said.

Marshall, 57, said he was urged to get in the race while he campaigned in the fall for reelection to the House, where he has served since 1992.

RICHMOND, VA - JANUARY 12: Del. Robert Marshall (R-Prince William) says he will run for U.S. Senate this year against former senator George Allen (R) on January 12, 2012. (Photo by Tracy A. Woodward/The Washington Post) (Tracy A. Woodward/THE WASHINGTON POST)

He said he planned to call Allen to officially tell him that he was entering the race. Others in the GOP race include Jamie Radtke, former chairwoman of the Virginia Federation of Tea Party Patriots; Hampton Roads lawyer David McCormick; and Bishop Earl Jackson.

Former governor Timothy Kaine is the almost-certain Democratic nominee in the contest to succeed retiring Sen. James Webb (D).

An Allen spokesman did not return a request for comment. Gov. Robert F. McDonnell (R), who endorsed Allen, said that Marshall is a principled legislator who has a core set of supporters but that it was too late to join the race. Allen has been running for a year.

“George Allen has a tremendous reservoir of support across the Republican spectrum,” McDonnell said. “I think he’d be difficult to beat.”

Marshall almost defeated former governor James S. Gilmore III to capture the Republicans’ Senate-race nomination at a 2008 convention to run against former governor Mark R. Warner (D). The margin was less than one percentage point.

But Republicans have decided to hold a primary this year, not a convention, and that’s expected to benefit Allen, who is well-known statewide.

To get on the ballot, candidates have until March 29 to collect 10,000 signatures from across the state, including 400 from each of the 11 congressional districts.

Although the most recent census figures were released last year, the General Assembly has not redrawn the boundaries of congressional districts for this year’s elections.

Marshall has asked Attorney General Ken Cuccinelli II (R) for an opinion on whether he should collect signatures from the existing districts or wait.

Virginia’s ballot requirements have come under scrutiny since former U.S. House speaker Newt Gingrich and Gov. Rick Perry (Tex.) did not qualify for the Republican presidential primary ballot.