WASHINGTON — Democratic Sen. Elizabeth Warren said she will forgo the chase for campaign money from the biggest donors in her quest for her party’s nomination for president. In an email to supporters on Monday, Warren of Massachusetts acknowledged that “this decision will ensure that I will be outraised by other candidates in this race.” But by making her move now as a proactive gesture, vowing to avoid any “fancy receptions or big money fundraisers only with people who can write the big checks,” Warren is betting that focusing on grassroots supporters will ultimately pay off by boosting her small-dollar donations as she battles a half-dozen or more rivals. Warren billed her new fundraising policy as a way of matching her actions to her calls for a sweeping change to the nation’s political systems.