By Alina Selyukh

WASHINGTON (Reuters) - The Republican National Committee said on Friday it raised $13.7 million in March, its biggest haul so far this campaign season.

The committee, an engine of Republican Party fundraising for races for the U.S. Congress, has been digging itself out of a multimillion-dollar debt and on Friday said it owed $9.9 million at the end of March -- $1 million less than a month earlier.

The committee started fundraising jointly this month with Mitt Romney, the party's likely presidential nominee. It had $32.7 million in cash on hand at the end of March.

In February, the RNC reported receiving $10.7 million in contributions and $26.7 million in cash on hand.

Its counterpart across the party line, the Democratic National Committee, received $7.1 million in contributions in February, with $21.2 million left in cash on hand at the end of that month. The DNC had $5.8 million in debt.

The RNC and the DNC are due to file their official March fundraising reports with the Federal Election Commission on April 20.

(Additional reporting Alexander Cohen; Editing by David Brunnstrom)