The Tea Party Patriots will host a phone conference with the Republican presidential candidates on Dec. 18, which will be followed by an online straw poll, they announced Tuesday.

Mitt Romney and Michele Bachmann have already confirmed they will attend, while Ron Paul has declined. The group, which claims 15 million members nationwide, touts itself as the largest grassroots Tea Party group in the country.

"This is a chance for candidates to make their case to the Tea Party, and for Americans across the country to find out who will represent and advance their principles in office,” said Tea Party Patriots co-founder Jenny Beth Martin. "Just as in 2010, candidates must prove to the Tea Party they will be fiscally responsible and protect the Constitution in the White House."

Invitations went out Friday. Each participating candidate will have 10 minutes to answer pre-submitted questions from Tea Party groups around the nation. Afterward, participants will vote for their preferred candidate. The group is working with the other candidates on scheduling and expects more will participate.