Today, Gov. Bob McDonnell finally endorsed a candidate in the GOP presidential primary. He had hinted all along that it would be a former or current governor and he waited until there was only one of those left: Mitt Romney. Monday former Gov. Jon Huntsman dropped out of the race and yesterday Texas Gov. Rick Perry exited the contest.

It has been reported that McDonnell will spend at least some time in South Carolina campaigning for Romney in advance of the primary there tomorrow.

But, will his endorsement really help the former Massachusetts governor? The answer is likely no.

Yes, under normal circumstances, McDonnell could help Romney in the commonwealth. But, only Romney and Texas Rep. Ron Paul made it onto the Virginia ballot. The rest of the candidates didn’t present enough valid signatures to qualify. So, Virginia’s GOP primary will likely be a footnote in this race, with the candidates devoting little to no time and money here. Therefore, McDonnell’s endorsement here won’t matter to the extent that Gov. Jim Gilmore’s did in 2000 (he helped save George W. Bush’s lagging campaign) and Gov. Tim Kaine did in 2008 (he was one of the first establishment folks to endorse Barack Obama and helped him overwhelming win Virginia).

Will McDonnell’s endorsement matter in other states? Again, the answer is likely no. Can you imagine a Republican governor from Mississippi making much of a difference in a campaign in Virginia? Generally, governors really only hold sway over their particular state.

In fact, the real assist McDonnell gave Romney today was to change the headline from “Romney’s bad debate performance” or “What is Romney hiding in his tax returns?” or “Romney loses lead in South Carolina.” The endorsement of a popular, establishment Republican can bolster the aura of Romney as the inevitable nominee. This can help keep ease the worries of the rest of the Republican hierarchy and change the negative narrative (at least slightly) with the national media and the blogosphere.

In a week of bad news for the Romney campaign, McDonnell’s endorsement is a bright spot, even if it won’t make much of a difference in the Old Dominion or South Carolina.