AUSTIN -

In a last-minute surprise, U.S. Rep. Steve Stockman has filed paperwork to challenge fellow Republican and incumbent John Cornyn for U.S. Senate in Texas.

Cornyn is the Senate's majority whip and had appeared likely to escape a major challenge from the tea party or other far-right factions.

But Texas Republican Party spokesman Spencer Yeldell said Stockman came in shortly before Monday's evening's filing deadline, withdrew his House re-election bid and formally challenged Cornyn.

"I think this is much more of a nuisance for Senator Cornyn than an actual worry. However, Steve Stockman will run an aggressive campaign albeith very unfunded and he'll serve as a lightening rod for movement conservatives who are unhappy with Senator Cornyn," said chair of the Rice University Political Science Department, Mark Jones.

Jones says Cornyn has more money and a strong conservative base.

Stockman was elected to a new House district outside Houston last year, but still should be a decided underdog in the race.

Previously, he served one term in Congress starting in 1994 -- and was perhaps best known for accusing the U.S. government of "executing" members of the Branch Davidian cult after the siege near Waco.

The two republicans will face off in the March 2014 primary.