Regardless of which party controls Congress, presidents must negotiate their agenda on an almost one-by-one basis with each member of Congress.

Congress is America’s federal parliament. It is divided into two chambers: the House of Representatives and the Senate.

The House of Representatives is composed of 435 representatives elected for a two-year term. Each American state elects at least one representative, but the total numbers elected in each state depend on their population.

On average across the country, 710,000 people elect one representative. This means California elects 53 representatives, while Wyoming elects only one.

The Senate is composed of 100 senators elected for a six-year term. Each state elects two senators regardless of population.

Every two years, Americans vote to (re-)elect all representatives and one-third of senators. In a presidential election year, the congressional election happens in conjunction with the presidential election.

According to the American presidential system, the president is head of state and head of the executive branch.

Because the people directly elect the president – through the convoluted Electoral College process – there is no confidence vote or any process according to which government gets formed within Congress. The president appoints cabinet members, who serve at the pleasure of the president – not Congress.

Congress is vested with legislative powers. All legislation must be proposed by a member of Congress and approved by a majority of both chambers.

The president is not a member of Congress and typically only physically goes to Congress once a year to deliver the State of the Union address, which is simply a speech and is not even followed by a vote.

The vice-president presides over the Senate. However, the vice-president does not have the right to vote unless there is a tie – which, with 100 senators, is possible.

The separation of powers in the American system is very strong: there is no link whatsoever between the executive and legislative branches. The famous American system of checks and balances, however, puts the Senate in a special position.

The Senate alone must confirm every nomination made by the president of an individual to a public office. While this sounds very similar to a “confidence vote”, it is nothing of the sort. Instead, it is considered a check on presidential powers, not a partisan/political tool.

Confirmation votes are often bipartisan and, for instance, no Senate majority would ever deny confirmation to cabinet members because of party affiliation. Similarly, the Senate must also ratify all international agreements negotiated and signed by the president.

Congressional incumbents are almost always re-elected. Time and again, when a new legislature begins, about 90 per cent of incoming members were there during the previous Congress. This is the “incumbency advantage”.

The incumbency advantage affects both the House of Representatives and the Senate. This makes it extremely hard for major swings in Congress’s majority/minority structure.

Even in years when there is a clear change of the congressional majority, these swings represent only about 50 or 60 seats changing party in the House of Representatives and five or 10 seats changing party in the Senate. This is equivalent to roughly 10 per cent of all seats in each chamber.

The Republicans currently have 61 more seats than the Democrats in the House of Representatives and 10 more in the Senate.

According to some, the Democrats have a shot at winning some ground in the Senate in November, with the likely outcome being either a 49-51 or a 50-50 split. In case of a 50-50 split, whichever party controls the White House will also control the Senate through the vice-president’s tie-breaking vote.

The problem with congressional politics is that partisan affiliation matters very little.

Just as voters determine each party’s presidential candidate through primary elections, congressional candidates in most states are also chosen by voters in primary elections. This has significantly decreased the power of political parties and has almost completely eliminated the concept of the “party line” in Congress.

In American politics, there is little-to-no party line or party discipline. Each member votes according to their own preference. This may sound great to those elsewhere who are irritated with partisanship and voting solely on party lines, but it creates insurmountable problems that largely do not exist outside the US.

Presidents negotiating their agenda with each member of Congress often means finding “pork” to add to every single bill, no matter how trivial or how important, in order to gain one single vote in support.

This is why, for instance, Barack Obama had so much trouble gaining congressional approval of his signature healthcare reforms in 2009 and 2010 – even though the Democrats controlled both chambers of Congress at the time.

A Democratic president can negotiate much more easily with a Democratic majority in Congress than with a Republican majority. But being in the majority does not mean automatically having the votes to implement anything.

The worst-case scenario, however, is split control between the presidency and Congress, especially the House of Representatives. Republican members have been particularly successful at waging war against Democratic presidents, as seen in the “Contract with America” revolution led by Newt Gingrich in the 1990s against Bill Clinton, or the more recent government shutdown led by the Republican congressional leadership.

The current Republican majority in the Senate is refusing to hold confirmation hearings – let alone a vote – for Merrick Garland, nominated by Obama to the Supreme Court. This is a clear partisan use of a process that is not meant to be partisan. It shows how contemporary American politics is devolving, and hints at what the next president will have to deal with.

This article was also published on The Conversation.

Dr Rodrigo Praino is a lecturer in Politics and Public Policy at Flinders University’s School of Social and Policy Studies and director of the Graduate Program in Public Administration online. He is an expert in American politics and elections and has published extensively in the field. His work has appeared in journals such as American Politics Research, Social Science Quarterly and Congress and the Presidency. His academic work has featured prominently in the international media, including the Washington Post, Politico, the Huffington Post, the Pew Research Centre and the Discovery Channel.