A day before the House is set to hold its historic vote on health care reform, President Barack Obama urged Democrats to remember the core principles of their party in his final public plea to them.

“Something inspired you to be a Democrat instead of running as a Republican. … This is one of those times where you can honestly say to yourself, doggone it, this is exactly why I came here. This is why I got into politics. This is why I got into public service,” Obama said in a Saturday speech to the House Democratic Caucus.

The president called the bill “the most important piece of domestic legislation since Medicare” and a “vast improvement over the status quo.”

He said he understood the pressure that some lawmakers were under to oppose the bill but urged them to do what is right over what may be politically popular. He used the words of former President Abraham Lincoln: “I am not bound to win, but I am bound to be true.”

Obama singled out two lawmakers who each received a standing ovation — Reps. Betsey Markey (D-Colo.) and John Boccieri (D-Ohio) — as proof that “good policy makes good politics.” Both represent swing districts and both recently came out in favor of the health care reform bill.

“I look at him with pride,” the president said of Boccieri.

Congressional Democratic leaders also talked up the importance of Sunday’s vote.

Speaker Nancy Pelosi (D-Calif.) told her Caucus that they are “on the verge of making great history.” And Senate Majority Leader Harry Reid (D-Nev.), the lone Senator at the meeting, said he was “happy to announce” that a significant majority of Senators have officially committed to use reconciliation to pass the overhaul.

Reid waited until Saturday to release a letter endorsed by the Senate rank and file — but that showed none of their signatures — intended to calm nervous House Democrats before their Sunday vote on the comprehensive Senate-passed measure and a budget reconciliation bill of fixes.

Sources said Friday that some Democratic Senators asked Reid to keep the letter private because they were uneasy about sticking their necks out before House action on the bill.

Several top administration officials attended Saturday’s Caucus meeting, including Health and Human Services Secretary Kathleen Sebelius, Office of Management and Budget Director Peter Orszag, Interior Secretary Ken Salazar and Transportation Secretary Ray LaHood.