When 44-year-old Carlos Romero Barcelo is sworn in shortly before noon Sunday as Puerto Rico's fifth elected governor, he will take charge of an economically depressed commonwealth that his party has pledged to convert into the 51st state.

That pledge was given immediacy on New Year's Eve by President Ford's announced plan to push for Puerto Rican statehood through congressional legislation.

But Ford's move has caught Romero's party - traditionally tied to the Republican Party - by surprise.

His pro-statehood New Progressive Party campaigned on a platform stressing economic stimuli to offset the lingering business depression here, rather than on the statehood issue.

The NPP pledged not to press for immediate statehood in an effort to woo voters traditionally loyal to the Popular Democratic Party, headed by Gov. Rafael Hernandez-Colon, which favors retaining the island's commonwealth status.

Romero's strategy worked. Even people in the small mountain towns that form the backbone of PDP support switched to the NPP in significant numbers and helped Romero win a 43,000-vote plurality over Hernandez. The NPP also picked up majorities in both houses of the legislature.

Much of the vote-switching, according to observers here, came from unionized workers embittered by the austerity policies of the Hernandez-Colon administration. The anti-Hernandez-Colon sentiment was said to be especially strong among organized government employees, who worked under a wage freeze with token pay increases from 1973 through 1976.

Another voting force that hurt Hernandez-Colon was the small but rapidly growing movement in favor of Puerto Rican independence.

Although the independentistas garnered fewer than 94,000 of nearly 1.5 million votes cast for governor in November, they made a big gain over the 1972 election, when pro-independence candidates polled 73,000.

The two main pro-independence parties, both of which put up candidates in November, are the Puerto Rican Independence Party, modeled after European Democratic Socialist parties, and the more militant Marxist Puerto Rican Socialist Party.

PIP president Ruben Berrios Martinez got 833,108 votes, a gain of 13,000 over the showing made by the PIP's 1972 candidate. The PSP, which boycotted the 1972 elections, received 10,797 votes for its gubernatorial candidate, party secretary-general Juan Mari Bras.

Hernandez-Colon's party has many independent sympathizers, and the party supports greater autonomy for Peurto Rico within the commonwealth relationship. So any increase in independentistas' votes tends to cut into the PDP total.

In 1972, Hernandez-Colon polled 659,000 - or 51 per cent - of the nearly 1.3 million ballots cast. But his 1976 total stayed practically level at 661,000 despite a surge of nearly 200,000 additional voters, for 45.3 per cent of the vote.

More than any other issue, the economic slump cost Hernandez-Colon the election. Most of Puerto rico's 3.1 million people are more worried about bread-and-butter issues than about the long-term problem of relations between the 3,435-square-mile island and the U.S. government.

The current economic slump started more than three years ago and has depened to the point where, by official estimates, the unemployed make up more than 20 per cent of an estimated labor force of 900,000. The U.S. unemployment rate is 8.1 per cent.

Puerto Rican economists estimate that at least a million more able-bodies adults have given up job hunting, so that unemployment actually may be twice the official estimate.

In his campaign, Romero stressed economic issues, including the need for recovery in the badly hurt tourism and construction sectors.

The island's construction industry has been in a tailspin since President Nixon clamped a year-long freeze on federally funded housing projects in January, 1973. The industry's troubles deepened because of overbuilding of luxury high-rise condominiums.

Tourism began falling at the beginning of the 1970s, coinciding with overbuilding of hotels and urban development that have taken away some of the island's allure as a tropical paradise.

Romero has promised to help Puerto Rico's tourist industry recover, but he has not yet detailed how he will do it. He campaigned against slot-machine gambling - one source of tourist dollars - saying that it tarnishes the luxury hotels' image.

Romero also pledged to resell to private investors the telephone and cargo shipping public-service corporations purchased by the commonwealth during the Hernandez-Colon administration. Romero repeatedly blasted the government purchases, saying they had badly weakened private business confidence in the Puerto Rican economy.

Now, Romero will be pressed to move fast to carry out his economic recovery promises.Throughout his campaign, he pledged to forgo any immediate drive to change Puerto Rico's relations with the united state until economic recovery was well under way.

Romero's soft-spoken campaign style, relying heavily on face-to-face, hand-shaking campaign forays into Puerto Rico's towns and farming districts, seems to have convinced many islanders of his sincerity in seeking to put the island's economy back on its feet.

But President Ford's surprise prostatehood annocuncement has placed the incoming governor on the spot. The more ardent pro-statehood supporters within Romero's party can be expected to press their leader for accelerated action toward statehood, sources said.

And if Romero heeds the pro-state hood pressure, it could cost him the support of many pro-commonwealth voters who backed him in November.

There has been no direct test of Puerto Rican sentiment for statehood versus commonwealth status or independence since an islandwide plebescite in 1967.

In that year, 60 per cent of those voting favored continuing the commonwealth relationship and 39 per cent voted for statehood. The independentistas boycotted the plebescite.

One key figure in any Washington lobbying this year for statehood will be the newly elected resident commissioner, Baltasar Corrada del Rio.

Corrada has gained admittance into the Democratic Party caucus in the U.S. House of Representatives, though his party is unofficially tied to the GOP.

The resident commissioner has no vote in the House but he can introduce bills and participate in committee voting.

Corrada, who belongs to the militantly pro-statehood wing to the NPP, says he is a longtime Democrat.

"I like the Democratic Party stand on social and economic issues and feel I can accomplish a lot more for Puerto Rico by siding with the Democrats," he said last November.