Presidential candidates Michele Bachmann, Tim Pawlenty and Rick Santorum are ready to board NOM's anti-gay marriage bus tour.

The National Organization for Marriage (NOM), the nation's most vociferous opponent of gay marriage, is launching its 4-day, 22 city Values Bus Tour on Tuesday.

Bachmann, Pawlenty and Santorum will be joined by Iowa Rep. Steve King, Texas Rep. Louie Gohmert, Family Research Council President Tony Perkins, Susan B. Anthony List President Marjorie Dannenfelser and NOM bosses Maggie Gallagher and Brian Brown on the 1,305 mile trip that begins in Des Moines and arrives four days later in Ames, Iowa as the Ames Straw Poll opens.

Notably absent is any representation or sponsorship by The Family Leader, the Iowa-based Christian conservative group headed by former Iowa gubernatorial candidate Bob Vander Plaats. Last month, the group's 15-point anti-gay, anti-gay marriage, anti-abortion rights, anti-pornography pledge drew widespread criticism and even the two GOP presidential candidates who signed the pledge ā€“ Bachmann and Santorum ā€“ expressed concern over some of its content.

ā€œPresident Obama has done virtually everything in his power to undermine the institution of marriage, including refusing to defend the bi-partisan Defense of Marriage Act signed into law by President Clinton. NOM is committed to ensuring that the next president is a strong and committed supporter of traditional marriage and will commit his or her administration to vigorously defending marriage in the courts, Congress and in the court of public opinion,ā€ Brown said in announcing the tour.