Mitt Romney has picked up another endorsement from Sen. John Thune, R-S.D., adding to his collection of prominent supporters.

Thune -- who has been a rising star in the Republican Party since he beat former Senate Majority Leader Tom Daschle, a Democrat, in 2004 -- is the second conservative leader to endorse Romney this week. Kelly Ayotte, a first-term senator from New Hampshire, endorsed him on Sunday.

Thune received a 100 percent rating from the American Conservative Union in 2010 and has a lifetime rating of just under 89 percent. The ACU has not yet rated Ayotte because she was just elected last November, but she holds solidly conservative positions on both social issues (she opposes abortion, same-sex marriage and gun control) and fiscal issues (she supports a balanced budget amendment and a permanent extension of the Bush tax cuts, and she opposes the Obama health care law and the Dodd-Frank financial regulatory law).

Romney is winning the overall endorsement race by a large margin, but it is endorsements from people like Ayotte and Thune that bolster his campaign the most, because they increase his credibility among the voter blocs that are the most hesitant to support him.

Many conservatives are loath to vote for Romney because of the moderate policies he pursued as governor of Massachusetts and his perceived inconsistency, and it is hard to say whether conservative endorsements will make these voters change their minds -- but they will certainly sway some voters who are on the fence.

Thune: Romney Has the Experience

Thune said that Romney has the needed experience to lead an economic recovery and would pursue fiscally conservative policies. His plans to revitalize the private sector and restore our country's fiscal health are drawn from his 25-year career as a conservative businessman, he said, according to The Washington Post. Washington could use these common-sense principles at such a critical time.

Ayotte took a different approach, calling Romney the most experienced and electable candidate rather than touting his conservative credentials.

There's one person in this field who is prepared to lead the United States of America, and that is Mitt Romney, she said in an appearance with Romney in Nashua, N.H., on Sunday. Most importantly, there is one person who I know will ensure that Barack Obama is a one-term president, and that is Mitt Romney.

Conservative voters who are willing to put perceived electability above ideological purity will probably be swayed by the influx of Romney endorsements. On the other hand, there are many voters who will refuse to settle for any candidate who offers less than unconditional support for conservative principles. Romney's poll numbers in the coming weeks, and his results when voting begins in January, will show how big those categories are.