Instead of asking conservatives to call home-state senators with pleas for votes against health care reform, the Republican National Committee is asking its membership to contact the offices of two Democratic senators deemed “critically important” to squashing any up-or-down vote on the reform bill.

U.S. Sens. Blanche Lincoln of Arkansas and Ben Nelson of Nebraska are two of three members of the Senate Democratic Caucus who have yet to say whether they will act to prevent a planned Republican filibuster of the Senate’s health care reform bill. (The third is Sen. Mary Landrieu of Louisiana.)

(Update: Nelson, citing a personal history of rejecting “efforts to obstruct,” has said he will vote to allow the bill to move forward.)

“[Senate Majority Leader] Harry Reid has scheduled a critical vote this Saturday night that will allow the Senate to take-up his bill,” the RNC wrote today in an e-mail that urges supporters to contact Lincoln’s and Nelson’s offices.

“This is the best opportunity there will be to stop President [Barack] Obama, Harry Reid and Nancy Pelosi from imposing their government-run health care plan on America.”

Speaking with reporters on Thursday, U.S. Sen. Tom Harkin predicted that the planned filibuster would fail because “no Democrat wants to be the one that stopped” health care reform.

Democrats, who currently control the Senate, will need 60 votes to overcome a filibuster. That means, in addition to holding their caucus of 58 votes together, the Democrats will need the support of independent Sens. Bernie Sanders of Vermont and Joe Lieberman of Connecticut.