* S&P 500 index falls as Trump impeachment rumblings increase
* Sterling gains on bets court ruling cuts immediate Brexit risk
* U.S. consumer confidence plunges in September (Updates with close of European markets; changes byline, dateline)
By Chuck Mikolajczak
NEW YORK, Sept 24 (Reuters) - A gauge of global stocks fell on Tuesday as calls for the impeachment of U.S. President Donald Trump grew louder, while the British pound firmed as the UK Supreme Court ruled Prime Minister Boris Johnson's decision to suspend parliament was unlawful.
After initially advancing, Wall Street's benchmark S&P 500 index retreated to session lows after U.S. Representative John Lewis, one of the most senior leaders, joined other Democratic lawmakers in calling for impeachment proceedings to begin against President Donald Trump.
The call came after Trump confirmed he held back aid to Ukraine but denied it was done to provoke an investigation of Democratic political rival Joe Biden, the former vice president. Biden was to deliver a statement calling for Trump's impeachment later on Tuesday.
House of Representatives Speaker Nancy Pelosi, a Democrat, was to meet later on Tuesday with Democratic lawmakers to consider opening impeachment proceedings.
"They are taking a huge risk here, as are the Republicans, digging in on both sides - it is interesting to see how this is going to play out," said Paul Mendelsohn, chief investment strategist at Windham Financial Services in Charlotte, Vermont. "Is it good for the market? – Probably not."
The Dow Jones Industrial Average fell 189.62 points, or 0.7%, to 26,760.37, the S&P 500 lost 27.24 points, or 0.91%, to 2,964.54 and the Nasdaq Composite dropped 118.87 points, or 1.47%, to 7,993.59.
The tone of the market had begun to weaken earlier in the session, with gains evaporating after weak data on U.S. consumer confidence and a speech by Trump to the United Nations General Assembly that called on nations to embrace nationalism and turn their backs on globalism. Trump also issued a message to China that he will not accept a "bad deal" in trade negotiations.
Markets had been optimistic about the trade tone after U.S. Treasury Secretary Steven Mnuchin said on Monday that he and U.S. Trade Representative Robert Lighthizer would meet with Chinese Vice Premier Liu He for trade talks in two weeks, after waning optimism on trade had hit the stock market on Friday.
A report showed that U.S. consumer confidence fell by the most in nine months in September, far more than expected, as Americans' economic outlooks darkened in the face of the U.S.-China trade war.
Investors have looked to U.S. consumer strength as a reason to be optimistic about the economy despite signs of weakness in other areas, such as manufacturing.
European shares closed near flat, as Brexit uncertainty and the U.S. consumer data helped fan persistent growth worries.
The pan-European STOXX 600 index rose 0.01% and MSCI's gauge of stocks across the globe shed 0.51%.
Sterling gained after the court ruling against Johnson, but uncertainty over a possible general election and the eventual outcome of Britain's impending exit from the European Union capped its rise.
Sterling was last trading at $1.2472, up 0.35% on the day.
The dollar index, which measures the greenback against a basket of six major currencies, fell 0.22%, with the euro up 0.19% to $1.1012.
Benchmark 10-year U.S. Treasury notes last rose 15/32 in price to yield 1.6576%, from 1.708% late on Monday.
Reporting by Chuck Mikolajczak; Editing by Dan Grebler