Each year in Albany, a multitude of bills on housing issues go through a familiar pattern: The Democratic-led Assembly, supported by affordable housing groups, passes legislation that would expand tenants’ rights and protections to varying degrees, including bills that would strengthen rent stabilization. Then the Republican-led Senate, backed by the New York City real estate industry, votes down the legislation or takes no action, often saying the bills would crimp economic development or encroach on property owners’ rights.

Now, with an increasingly narrow Republican majority in the Senate and the retirement of Senator Joseph Bruno, who led the Republican side for 13 years, the recurring rhythm of real estate legislation—and the political alliances that go along with it—could shift substantially.

With Democrats two seats shy of Senate control, a number of vulnerable Republican incumbents, and a national presidential election that could bring strong turnout for Democrats statewide on Nov. 4, long-dormant real estate legislation could be front and center under a new Democratic Senate. To name a few issues: rent control; a real estate incentive program, regulations limiting the ability of property owners to privatize their Mitchell-Lama affordable housing complexes; and tax breaks for residential construction.

“I have about 121 house bills, and probably half of them are in rent regulations, and they’re as old as me,” said Assemblyman Vito Lopez, chairman of the Assembly housing committee. “If the Senate goes Democrat, the chances of them moving those bills will greatly increase.”

Topping the list of real estate issues potentially up for grabs is high-rent vacancy decontrol, the law that allows landlords to free vacant apartments from rent regulations once the rent passes a cap of $2,000 a month. Just a decade ago, Mr. Bruno came close to eliminating all such rent regulations, but now many Senate Democrats pledge to raise the cap or remove it entirely.

Moving the Democrats into power has become a prime focus of tenant groups, and housing affordability advocate Michael McKee, who serves as treasurer of the Tenants Political Action Committee, said his efforts are aimed entirely at the Senate.

“Everything rides on the Senate changing hands,” said Mr. McKee, who added that his group has been assisting in the campaign of Joseph Addabbo, a Democratic councilman from Queens seeking to unseat Republican Senator Serphin Maltese.

Among Senate Democrats, Liz Krueger, the ranking Democrat on the Senate’s housing committee, said the vacancy-decontrol cap is a major issue, one that she would seek to remove completely, though she said she’s open to raising its limit.

“The conference supports rent stabilization, and recognizes that we have a housing crisis in the city of New York,” Ms. Krueger said.

In May, Ms. Krueger was joined by the bulk of the Democratic conference in the Senate—26 of the 29 other Democrats—in writing a letter to Mr. Bruno pledging support for the removal of the vacancy-decontrol cap entirely.

The letter, which aligned the members with Assembly Democrats on the issue, was also notable for one of the three who did not sign it: Malcolm Smith, the Democratic Senate minority leader.

Mr. Smith’s lack of open support for some of the rather liberal housing policies espoused by his colleagues is suggestive of a broader ambiguity as to how the Senate Democrats would actually vote if they came to power.

The real estate industry tends to be a major source of campaign funds for elected officials, particularly those in the majority party—a temptation that many in Albany say could prove difficult to resist, particularly if the margin of control is slim.

“It’s going to be a fascinating test of politics and character to see if we stick with the people who have helped us take the majority, or if we realign with people who have essentially opposed us all along the way,” a Democratic senator said, speaking, respectively, of tenant advocates and the real estate industry.

The main advocacy group of the real estate industry, the Real Estate Board of New York, is still strongly in support of Senate Republicans, but it does not seem entirely pessimistic about a Democratic-led Senate.

“We’ve had conversations with Malcolm Smith; he’s indicated that the Democrat leadership, if they should get the leadership, will act responsibly and will take into consideration the concerns of the business community,” said Steven Spinola, president of REBNY. “He personally has a fairly good track record along those lines.”

Also unclear is if, or how, the Assembly would change its approach to real-estate-related legislation, as up until now it has passed numerous liberal bills that were always expected to die in the Senate, including a bill repealing high-rent vacancy decontrol, which was passed by the Assembly in May.

“What I worry about is that if that check isn’t there anymore,” Mr. Spinola said. “Some of the bills that pass the Assembly—which up till now everyone knew would never become law—will they continue to pass such legislation?”

Mr. Smith could not be reached for comment.

But the real estate industry is clearly looking to him to provide a moderate check on the rest of his conference should the Democrats claim the majority and should Mr. Smith be their leader, claiming that the ability to take and then hold power could depend on it.

“Based on the legislation that’s been introduced by some of the members … they would seek to overturn all the legislation enacted under the Pataki administration,” said Joseph Strasburg, president of the Rent Stabilization Association, a powerful landlords’ group.

“I think that Malcolm is attempting to be the voice of reason, because clearly you would recognize that if that’s the direction the majority is going to go to,” Mr. Strasburg said, “then the people who provide resources in the real estate industry clearly will not assist the Democrats in their endeavor to take over the Senate.”