THERE'S little wonder a lot of political leaders are feeling vulnerable in Washington State.If a controversial measure - Initiative 553 - is approved by the states's voters in November elections, many politicians, including United States House of Representatives Speaker Thomas Foley, might have to start thinking about getting a new job outside public office. Initiative 553 is the product of a grass-roots organization called "LIMIT an acronym for "Limitation Initiative Mandating Incumbent Terms." Under the measure, lawmakers in the state legislature would be limited to no more than 10 consecutive years in office. The state's governor and lieutenant governor could not serve more than two consecutive terms of four years each. The main controversy, however, involves federal officials. Specifically, US senators would be limited to two six-year terms and US representatives to three two-year terms. No federal officeholder could serve more than 12 consecutive years. Somewhat similar, but less stringent, measures were approved last year in California and Oklahoma regarding state elected leaders; meanwhile, Colorado last year went a step further and provided for some limitations on federal officials. "We are just very confident that this measure is going to pass," says John Burick, communications director for LIMIT, based in Tacoma, just south of here. Much of the financial backing for LIMIT has come from out of state - the Citizens for Congressional Reform, a term-limit group based in Washington, D.C. "We did a poll earlier this year and found voter support at about 68 percent in favor of the measure," says Mr. Burick. Indeed, the petition to put the measure on the ballot here drew 255,000 signatur es. According to informal estimates, term-limitation plans are now being considered in 22 states; actual statutory measures are expected on the ballot in at least 17 states in 1992. Groups that sought to keep Initiative 553 off the November ballot or wanted a preelection court test on the constitutionality of the measure include the League of Women Voters, Common Cause, and organized labor. Many of these groups, in fact, had supported a legal challenge before the state Supreme Court. The high court, however, declined to bar the November vote or render a decision on the constitutionality of the initiative. According to Jean Womer, an official in the state election office, a "simple m ajority" of voters would be enough to approve the initiative. Critics question the degree of public support for Initiative 553. "My understanding is that the measure has broad support, but not deep support," says Edward Younglove, the attorney who filed the unsuccessful challenge to the initiative before the state high court. "When people are confronted with what this measure really means, then support dissipates," says Mr. Younglove. That said, political leaders here in a state noted for letting its federal officials serve for decades remain understandably nervous. If the measure is enacted and were to be subsequently upheld by the courts, a large part of the state's political leadership might find itself seeking new employment in the next few years. Those threatened by the measure include Speaker Foley and Gov. Booth Gardner. The measure would provide a "grandfather" clause for legislative incumbents meeting the term limits, including Foley, a 21-year legislative veteran; but the clause would only allow such incumbents to serve one additional consecutive term. "Term limitation is a way for private citizens to regain control of their government," insists Mr. Burick.