In principle, 50 votes is enough to win a Senate vote but the "filibuster" rule means that 60 votes are needed. Weirdly, 50 votes are enough to change the filibuster rules, but those Senators "like their ancient honorable traditions" and for decades those rules have never been changed without bipartisan support. In olden times filibusters were relatively rare and sometimes required that a Senator remain standing and talking for hours with no bathroom breaks allowed. Nowadays, essentially EVERY bill of any importance is automatically "filibustered": Nobody needs to stand and orate for hours, a simple fart by Mitch 'Moscow Mitch' McConnell suffices.
Therefore saying the Dems control the Senate when they have only 50 votes is simply wrong. It WOULD be enough if they voted to amend their "ancient honorable rules" but they don't. This is one reason statehood for DC and Puerto Rico is urgent: as of now 51 Senators can stop a rule change; with 52 states it would take 53.
So: To be "in control" the Ds need 60 votes. Or support from some of the Rs. This is hard to get.
In olden times some Republican Senators were honest good-spirited people who might vote their conscience. A Republican who thought gun control was a good idea might vote for it. Not any more! EVERY Republican Senator almost ALWAYS votes exactly as instructed by Charles Koch or the NRA or whoever is running their party. Think about that: On many substantial issues (healthcare reform, guns, women's rights, spending) Senate votes are EXACTLY on party lines. Would that be likely if all Senators actually voted their conscience?
Check the table near the middle of this Wikipedia page,
Party divisions of United States Congresses.
The last time the Ds had 60 Senators was the 95th Congress under Jimmy Carter 43 years ago. The 111th Senate (first two years of Obama) is shown as 56-58 with an asterisk. If you study the asterisk you'll see that they would have had 60 votes beginning April 28 if you count Arlen Specter, Al Franken and two independents. But you can't count Al Franken: The Rs pretended there was some controversy about his close election and he was not seated until July 7. By that time Edward Kennedy was gravely ill; the Rs did NOT extend the courtesy of "matching missings" and Kennedy had to be literally wheeled in from his death-bed to the Senate Chamber to cast a 60th vote. IIRC that happened at least once, Kennedy casting a key 60th vote for Obamacare. Kennedy soon died, but the Ds did have 60 votes for 4 months later. And that was the end of that.
But they didn't even have 60 real votes during the brief period that Al Franken was allowed to vote and Ted Kennedy could be wheeled in from his death bed. The DINO Joe Lieberman didn't represent actual PEOPLE: It was always clear that he was "the Senator for the Insurance Companies." A real health-plan would require reducing insurance industry profits, and that NEVER had more than 59 votes.
Some of the Repugnant Senators were smart enough to understand that Obamacare could be improved with their help. But EVERY SINGLE Repugnant toed the line and voted as Charles Koch instructed them.
And even with 60 votes, the Rs can delay and delay in various ways. During the brief period under Obama that the Ds had 59 votes or so, they dedicated much time and political capital just to Obamacare. There simply was not enough time to do more.
Today, the Ds have nowhere near 60 votes. They only have 48 unless you count Manchin and Sinema.
And even if they had 60 votes — or changed the rules — the Supreme Court might overturn any legislation. This wasn't a problem in olden days, when at least 5 (usually 8 or 9) of the Justices were righteous and good-spirited people who wanted to follow the law or achieve what was best for America or its people. Today only 3 or 4 of the Justices fit that mold. Even to get to 4 you have to include John Roberts, who started his career as a political hack in the mold of Karl Rove.
Sorry. When the liars tell you the Ds have or had control, it's just another lie.