Steve Helber, File, Associated Press
FILE - In this April 6, 2011 file photo, Del. Robert Marshall, R-Prince William, debates on the House floor at the Capitol in Richmond, Va. Marshall announced his candidacy for the Republican nomination for US Senate.

RICHMOND, Va. — Conservative Republican Del. Bob Marshall is challenging a former Republican governor for a U.S. Senate seat this year for the second time in four years.

Marshall said Thursday he will announce his plans and file federal campaign papers next week. He has already created his own campaign organization in Virginia. Marshall registered his committee, Bob Marshall for Senate Inc., with the State Corporation Commission on Monday.

He joins a field of four candidates led by frontrunner George Allen, a former governor trying to win back the Senate seat he lost six years ago to Democrat Jim Webb. Tea party leader Jamie Radtke, E.W. Jackson and David W. McCormick are already in the June primary.

Marshall narrowly lost the nomination in a statewide GOP convention in 2008 to former Gov. Jim Gilmore. Gilmore lost a lopsided election later that year to Democrat Mark R. Warner for the seat retiring Republican Sen. John W. Warner relinquished after five terms.

Allen's campaign declined to comment. A telephone message left with Radtke's campaign was not returned Thursday afternoon.

Marshall, one of the General Assembly's most outspoken social conservatives and abortion foes, faces an immediate challenge matching $2 million Allen had on hand at the start of the year after raising $1 million the final three months of 2011. The GOP Senate primary is June 5.

The race is among the nation's marquee contests as Republicans strive to erase a narrow Democratic majority in the Senate. The frontrunners from both parties are former governors, and the race plays out in Virginia, a presidential battleground that Barack Obama won in 2008 — the first Democratic presidential win in the state in 44 years.

Leading the Democratic field is Tim Kaine, who spent his final full year in the governor's office in 2009 moonlighting as Obama's handpicked Democratic National Committee chairman. Kaine left the DNC post in April to begin his Senate campaign.

Marshall's entry into the GOP field is less likely to affect Allen than the others, said longtime GOP strategist and former Allen adviser Christopher J. LaCivita.

"There is a finite group of the electorate in this primary that you could call the anti-Allen vote. Someone else getting in the race cuts up what was already a fairly small pie into smaller pieces," LaCivita said.