Sen. Pat Roberts (R-Kan.) has become the 14th GOP senator to sign on to the Defund Obamacare effort.

Roberts joined the effort, led by Sens. Mike Lee (R-Utah) and Ted Cruz (R-Tex.), on Wednesday, spokeswoman Sarah Little confirmed.

Defund Obamacare is a pledge to oppose any government funding that includes funding for President Obama's signature legislation.

Roberts's decision is a win for supporters of the effort, who began to apply pressure to him and other undecided senators this week. Roberts was one of a dozen targeted by new online ads from Tea Party Patriots and For America -- two groups that have joined to launch a Defund Obamacare tour next week.

Despite these efforts, some Republicans have already dismissed the effort, saying it's a poor strategy and that risking a government shutdown would wound their party and risk economic consequences.

Two other conservative groups -- Heritage Action and the Senate Conservatives Fund -- are also trying to recruit or convert GOP senators. SCF has launched ads against Sens. Lindsey Graham (R-S.C.), Richard Burr (R-N.C.), Lamar Alexander (R-Tenn.) and Thad Cochran (R-Miss.).

The effort would need 41 GOP senators to filibuster a government funding bill.