What are the grounds for impeachment?
They will likely be piling up rapidly. President Trump did use Day 1 to advise the CIA that the United States should have stolen all of Iraq's oil. But here is a place to start. We already have a president who is violating two clauses in the U.S. Constitution, one forbidding any gifts or benefits from foreign governments, the other forbidding the same from the U.S. government or any U.S. state. This is the result of Donald Trump refusing to separate himself from major business interests as past presidents have done. Those interests will also inevitably involve Trump in violating the STOCK Act which forbids the use of non-public government information to make a private profit.
Article II, Section 1 of the Constitution states: "The President ... shall not receive ... any other emolument from the United States, or any of them." This means that the President cannot receive personal financial gains from the United States government or from the governments of any of the 50 states while he is president. This restriction is absolute and cannot be waived by Congress. Trump is already in violation of it and will be more so with every law, rule, regulation, enforcement, or lack thereof that his subordinates, Congress, or any agency of the federal government enacts to the benefit of Trump's businesses and possessions.
For example, Trump's lease of the Old Post Office Building violates an explicit clause in the General Services Administration lease contract which states: "No ... elected official of the Government of the United States ... shall be admitted to any share or part of this Lease, or to any benefit that may arise therefrom." The GSA's failure to enforce that contract is an unconstitutional benefit to Trump.
Or, to take a state-level example: since 1980 Trump and his businesses have garnered "$885 million in tax breaks, grants and other subsidies for luxury apartments, hotels and office buildings in New York." Continuing or increasing those subsidies puts Trump in violation of the Constitution.
Article I, Section 9 of the Constitution says that "no person holding any office of profit or trust under [the United States government], shall, without the consent of the Congress, accept of any present, emolument, office, or title, of any kind whatever, from any king, prince, or foreign state." This is essentially the same ban as above, but applied to foreign governments.
The Trump Organization has licensing deals with two Trump Towers in Istanbul. Trump himself says, "I have a little conflict of interest, because I have a major, major building in Istanbul." China's state-owned Industrial and Commercial Bank of China is the largest tenant in Trump Tower. It is also a major lender to Trump. Its rent payments and its loans put Trump in violation of the Constitution. Foreign diplomats have begun shifting their D.C. hotel and event reservations to Trump International Hotel. The Embassy of Kuwait was reportedly pressured by the Trump Organization to do so. Pressured or not, Kuwait's business at a Trump hotel puts Trump in violation of the highest law of the land.
In November, there were reports (denied by Trump) that Trump had asked the president of Argentina for help with a building permit in Buenos Aires. Whether he did or not, and whether he receives that help or not, President Trump will be frequently granted or denied similar approval for his business ventures from numerous foreign and domestic governments.
Why punish a successful business man?
We can set aside the legality and morality of Trump's business success, and the question of how successful he has been. A campaign to impeach him for his violations of the Constitution can hold the position that Trump is perfectly welcome to keep all of his businesses and loans. He just cannot simultaneously hold an office in which they create gross violations of the U.S. Constitution. Past presidents have sold off their assets or placed them in a blind trust. A blind trust would not, however, be blind for Trump who would inevitably learn of the approval of new towers or the sale of properties. Selling (and using a truly blind trust to do so) was Trump's only option other than not being president. He chose not to take his only Constitutional choice.
Is this partisanship?
A great many people do anything political for partisan reasons. As I'm unable to put an end to that, it is inevitable that people will favor or oppose impeaching Trump for partisan reasons. But they need not. The above charges against Trump are unprecedented. They should apply to him and any future presidents who engage in the same abuses, regardless of party. Someone who voted for Trump as a way out of corruption should want him impeached as much as someone who voted against him for the same reason. Trump is now the worst possible "insider" -- using public office for personal greed.
Is this morally worse than Secretary of State Hillary Clinton taking Saudi government and Boeing funds into her family foundation, and then working to waive legal restrictions on Boeing selling weapons to Saudi Arabia -- weapons now being used to slaughter innocents? Some will think so and others not, largely along partisan lines. Personally I'm in favor of impeaching Clinton, Obama, and George W. Bush right now and imposing the penalties of a bar on holding future office and a denial of retirement benefits. But those efforts are simply not the same priority today as halting the presidency of the current president.
When I advocated for impeaching Bush I explained that if he was not held accountable his successors would expand further the abusive powers he had expanded. When I argued that Obama was in fact doing this and should be impeached, I was generally called worse things than partisan. But the longer presidents are allowed to act without a check on their powers, the more they will expand and abuse them. Numerous government officials and members of Congress would best serve the world by resigning. But the place to start is with an unprecedented and unique form and level of corruption in the single highest office in the land.