Dennis - A very basic US Civics 101 here. There are three branches of Federal Government. They are supposed to "check and balance" each other so no one segment runs amok in a mad power-grab. (ahem. Let's not go there right now.)
1) The Supreme Court - Rules on legalities and interprets the Constitution and Case Law (including criminal appeals). Once appointed, the Judges have jobs for life. Thus, it's important who is in power at the time a vacancy comes up on the Court, as once they are seated as Judges, they are the final word and have the job for a very long time (usually). So the powers that be at the time of that vacancy try to pack the court with judges who, shall we say, share a similar outlook as regards the interpretation of the law.
2) The Legislative Branch ("The Congress") consists of two bodies, the House of Representatives and the Senate. Each State gets two Senators. The House is done by population count - each state gets a number of seats based on proportional representation based on the every-ten-years Federal National Census. The Congress, collectively, is tasked with passing laws. IMPORTANT BIT - The size of a state's total congressional delegation determines the size of its representation in the U.S. Electoral College.
It is actually the College who elects the Pres/Vice Pres. (No, the USA is not a direct democracy. By the numbers, Hillary Clinton had more popular votes in the last presidential election, but the game was well played and Trump won states that had more electoral college members, so he was elected president with fewer popular votes than the loser. It has happened before.)
3) The Executive Branch is headed by the President. It is supposed to handle the actual running of the country (more or less). It is his job to take the mandates of the Congress and make them work. He has veto power over bills passed by Congress, but his veto can be overridden if enough of Congress votes to do so. The Pres/Vice Pres are elected to four year terms. (Two term limit.) While the Pres has some power to issue Executive Orders to manage operations of the federal government (and that have the force of law), those Orders can be reviewed and negated by the Supreme Court.
Mid-term elections happen between the every-four year elections when the Pres/VPres spots are up for grabs. All 435 members of the House of Representatives are up for election every two years. Members of the Senate serve staggered six-year terms, so 1/3 of the Senate is up for grabs at the mid-term election.
The USA has, for all realistic purposes, only two viable political parties - Democrats and Republicans. Once Upon A Time Democrats were thought of as the party of the common people, and the Republicans were the party of big business. How true that still is, or ever was, is open to debate. But I digress....
"The House and Senate are equal partners in the legislative process—legislation cannot be enacted without the consent of both chambers. However, the Constitution grants each chamber some unique powers. The Senate ratifies treaties and approves presidential appointments while the House initiates revenue-raising bills. The House initiates impeachment cases, while the Senate decides impeachment cases. A two-thirds vote of the Senate is required before an impeached person can be forcibly removed from office." Only the Congress has the power to declare war. It hasn't done so since the Pearl Harbor attack launched us into WWII. (The other military campaigns, like Korea, Vietnam, Iran/Iraq/Afghanistan? Long story. But they were not officially "wars" as such.
https://en.wikipedia.org/wiki/Declaration_of_war_by_the_United_States )
Bottom Line - IF you have a Republican president like, say, Trump, and a majority of the members of the Congress are of the same political party, the Pres can pretty much get whatever he wants done. The opposing party won't have enough votes to stop him. They might be able to get it pulled into review by the Supreme Court. But if the sitting pres has packed the Court, well, the outcome is unclear.
If, say, the Democrats take control (a majority of seats were voted for by the public to Democratic candidates) in the House/Senate, they can keep Trump from having legislation passed. It can be proposed by a Republican, but the Democrats can block it. Trump would then have to negotiate, either directly or through Republican member of Congress, deals to get what he wanted. Which might be nigh well impossible. Or he'd have to agree to something near and dear to the Democrats in return for their lack of obstruction of something near and dear to him/the Republicans.
Also, as someone else has said, if you get a big enough number of the opposing party into the Congress, impeachment can potentially be the result if there are grounds for doing so.:
"The U.S. Constitution provides impeachment as the method for removing the president, vice president, federal judges, and other federal officials from office. The impeachment process begins in the House of Representatives and follows these steps:
The House Judiciary Committee holds hearings and, if necessary, prepares articles of impeachment. These are the charges against the official.
If a majority of the committee votes to approve the articles, the whole House debates and votes on them.
If a majority of the House votes to impeach the official on any article, then the official must then stand trial in the Senate.
For the official to be removed from office, two-thirds of the Senate must vote to convict the official. Upon conviction, the official is automatically removed from office and, if the Senate so decides, may be forbidden from holding governmental office again. The impeachment process is political in nature, not criminal. Congress has no power to impose criminal penalties on impeached officials. But criminal courts may try and punish officials if they have committed crimes. The Constitution sets specific grounds for impeachment. They are “treason, bribery, and other high crimes and misdemeanors.” To be impeached and removed from office, the House and Senate must find that the official committed one of these acts."
http://www.crf-usa.org/impeachment/high-crimes-and-misdemeanors.html
