Montgomery County Del. Ida Ruben was having a quiet vacation in Ocean City, Md., during the summer when a little newspaper item caught her eye: Her state senator, Stewart Bainum, was thinking about running for a different office.

"I knew what I would do as soon as I saw it," said Ruben, a Democrat and chairman of Montgomery's House delegation. She had already scheduled her reelection fund-raiser and "the day before the invitations went to the printer, I was able to catch it," she said. She deleted "House of Delegates," and invited her guests to spend $35 to "Send Ida Ruben to the State Senate."

Ruben's sudden decision to seek a higher rung on the political ladder is part of what observers are calling Maryland's most sweeping political upheaval in at least two decades. Even though there is still a full year to go before the November 1986 elections, at least half of Maryland's congressional delegation, half of Montgomery's officeholders and half the Baltimore area's top political leadership have announced they will be seeking different jobs next year.

Prince George's is the only major jurisdiction unlikely to be affected by large turnover. Although all elected offices there will be up for grabs, as they are every four years throughout the state, most Prince George's incumbents are expected to seek the same offices.

"Next year is going to be a watershed year, because it could determine the next generation of political leaders," said Lanny Davis, Democratic national committeeman for Maryland, and a potential candidate for Montgomery County's congressional seat.

Said Victor Crawford, a former state senator and potential candidate for the same seat, "It will be an all-out bloodbath in 1986. There are so many strong candidates and there is no way you can keep the statewide races out of the local races, or the local races out of the statewide races. Nobody knows which way to turn."

The frenzy began in earnest in September with the announcement by Republican Sen. Charles McC. Mathias that he would retire, ending a quarter-century career in Congress and igniting a chain reaction of ambitious political leaders from Congress to the county courthouses.

Two members of Congress, Michael Barnes and Barbara Mikulski, as well as Gov. Harry Hughes and Baltimore County Executive Donald P. Hutchinson, are all expected to vie for the seat, setting off crowded and contentious races to replace all of them.

Next year had been expected to be volatile anyway, given the retirements of Rep. Marjorie Holt, Republican of Anne Arundel, and Montgomery Executive Charles Gilchrist, who is entering the ministry, and the replacement of the three top state officeholders.

One of those, Hughes, is completing a two-term limit set by the Maryland Constitution. His lieutenant governor, J. Joseph Curran, is running for attorney general. The incumbent attorney general, Stephen H. Sachs, has long made clear his plans to run for governor, in a contest where he will likely face the well-entrenched mayor of Baltimore, William Donald Schaefer.

Rep. Parren Mitchell, a Baltimore Democrat, added to the uncertainty in his city and the state by announcing his retirement within days of Mathias.

In addition, the powerful leadership of the House of Delegates will likely be gone. House Speaker Benjamin Cardin is currently a candidate for governor, and House Minority Leader Robert Neall, one of the most influential Republicans in Annapolis, is making a bid for Holt's congressional seat.

"You've got House of Delegate people who want to move up, and they're entitled to move up. People in the state Senate want to move up. People who work as precinct chairman want to move up to local office or the General Assembly," Crawford said. "The pressure builds. As long as Mathias was there, it kept the cork in. But his leaving popped the cork and everything exploded behind him."

In Montgomery County, the reverberations will be particularly acute with the loss of two of the county's most popular officeholders, Gilchrist and Barnes. Barnes' departure has spawned a field of nearly a dozen potential contenders for his seat and in the words of one of them, Montgomery councilwoman Esther Gelman: "Not a day goes by when I don't find out about some wonderful new candidate."

These candidates in turn will leave a vacuum that activists from the lowest levels of involvement will seek to fill. Bainum, for example, is one of those expected to seek the Barnes seat. Two of the three delegates in his Silver Spring-based district, Ruben and Diane Kirchenbauer, are already planning their respective assaults on his seat, creating openings in the House of Delegates that at least seven Democrats would like to fill. Among the hopefuls are three precinct chairman, a Takoma Park City Council member and a leader of the county's Young Democrats.

"The 'what if' game of Montgomery politics has been moving along in high gear," said Thomas Stone, the county's lobbyist in Annapolis. "What if . . . Stewart Bainum runs for Congress, and what if . . . he doesn't? It's like a heavyweight fight in the early rounds and they're checking each other out. We'll get on to the real fisticuffs later. So far there've been a few jabs here and a few jabs there."

Already, county observers have noticed a certain coolness in interpersonal relations. Said Kirchenbauer of Ruben, "I guess nothing's surfaced between us since she hasn't spoken to me since July."

Replied Ruben: "We speak when it's appropriate."

Montgomery legislators already eager to impress their constituents are filing a large number of local bills for the 1986 legislative session -- 70 so far this year, two months before the session begins, compared with 35 last year.

Also, said Del. Mary Boergers, another Montgomery Democrat interested in a state Senate seat, "People are playing out their campaigns in the delegation meetings, which is extending the length . . . . Picture-taking will be interesting. Every time there's a ribbon-cutting there'll be all kinds of jockeying for position."

The impact in the Baltimore area also will be enormous, with the possibility of two new congressmen, a new mayor and a new county executive. Howard County will be led by a new administration next year as well, when Executive J. Hugh Nichols ends his second term, the limit allowed there. He recently changed his party affiliation to the GOP in preparation for a run for the governorship.

With the turmoil in Baltimore County, Baltimore City and Montgomery County -- three of the state's major population areas -- political veterans expect that Prince George's County, another heavily populated jurisdiction, will emerge as a key battleground in the statewide races next year. The county, favored with an experienced political network and a large body of committed Democratic voters, is also the only one of the largest subdivisions not offering up a favorite son in the U.S. Senate race, leaving party workers available to line up behind a candidate from somewhere else.

The one group that appears least likely to benefit from the changes is the group that sparked it all: Maryland Republicans. The exit of their longtime standard-bearer, Mathias, has left them with a void that recruitment efforts have been unable to fill at the top of the ticket. At the bottom, the Democrat's 3-to-1 registration advantage have left Republicans with few viable races.

However, state GOP Chairman Allan Levey said that Republicans believe they will benefit from the potentially bitter Democratic primaries.

"I can say that when Mathias first announced his retirement, I was disappointed," Levey said. "However, when you look at what happened since then, his leaving the political scene of Maryland has created a number of open seats throughout the state, which can help the Republicans."

For example, Republican Del. Connie Morella may face one little known primary challenger in her bid for the 8th District congressional seat being vacated by Barnes, while perhaps a dozen Democrats will fight it out for their party's nomination.

Democrats and Republicans say the upheaval facing the state is unlike anything Marylanders have seen since 1966.

That year the Supreme Court's one-man, one-vote ruling forced a historic expansion of the General Assembly that permanently shifted power from rural areas to the urban and suburban.

"That caused a major change away from the Old Guard. At the time you had nine senators from the Eastern Shore and one senator from Baltimore City and one senator from Montgomery and one from Prince George's," said Del. Timothy Maloney (D-Prince George's). "The 1966 election saw the first strong suburban candidate for governor [Carleton Sickles] and all the 'shiny bright' reform candidates. That was the last major shift in politics."

Sickles, also a former congressman at large from Montgomery County, said, "It had a tremendous impact. Many people involved in that campaign came to prominence then. Mayor Schaefer was my Baltimore city chairman. Sachs. Steny Hoyer [Democratic congressman from Prince George's] was on my ticket."

Hutchinson, 39, noted that he and Mikulski, 49, also got their start in politics during that era. He added, "The reason that you have 1986 becoming the crucial year is, it is really a natural fallout of 1966 and the early 1970s . . . . It's a generation that's politically come of age. We all got involved at the same point in time."

At this point, observers say it is too early to speculate on the long-term impact. "It's very hard to place money on a horse when you don't even know how many horses are in the race." said Crawford.

But, promises Davis: "It's going to be the best spectator sport. They ought to be selling tickets to watch these things unfold. It will be pure entertainment."